AbstractPoylmer electrolyte membrane (PEM) fuel cells are a promising technology for power supply. They are quiet and efficient with low to zero emissions. At Fraunhofer Institute for Solar Energy Systems ISE, we developed a PEM fuel cell system within the Project REnerSta. The fuel cell system is part of a modular autonomous power solution using solely solar radiation and wind as energy sources. Short time storage of the fluctuating energy is realized by using Lithium Ion batteries while the combination of electrolyzer, hydrogen storage and fuel cell system provides long time storage capability. Efficiency of the fuel cell system was optimized by analyzing the energy flows at different operating points. Experimental results showed an efficiency of 44.7 % referenced to the lower heating value of hydrogen and including fuel cell losses, converter losses and BOP power demand while delivering approx. 1400 Wel. Additionally, influences of changing ambient temperature and humidity were considered.